Electronic control system for selfbalancing measuring instruments



A ril 11,1939.

F. B. MWLAREN. JR

- ELECTRONIC CONTROL SYSTEM FOR SELF-BALANCING MEASURING INSTRUMENTS Filed NOV. 15, 1937 mvsmon. FRED 5. MA cLARE/V, (/19. I

ATTORNEY.

Patented Apr. 11, 1939 UNITED STATES- PATENT orslcs aisasss mo'momc coN'raor. SYSTEM Foa snarnsumcmo MEASURING INSTRUMENTS I Fred B. MacLaren, Jr., Waterbury, Oonn., assignor to The'Brlstol Company, Waterbury, Conn., a

corporation oi Oonnecticut Application November .15, 1937, Serial-No. 174,615 A a 8 Claims.

This invention relates to electrical control systems, and more particularly to an electronic.

including a motor and motor circuit for restoring a balanced condition andwherein no electrical contacts are involved. 7 Another object is to provide a control system which shall be especially sensitive to changes in the controlled condition, and which embodies relatively simple electrical circuits and electronic devices.

A iurther object resides in the provision oi a control system oi the nature specified, embodying a balancing motor which may be given the advantageous characteristic oi dynamic braking. A still further object is to secure speedy response to an unbalanced condition, which response varies directly with the degree oi unbalance and any tendency to overshoot the balanced position being substantially eliminated.

The single drawing is a diagrammatic representation oi the novel control system, embodying anielectrical circuit including electronic devices and a self-balancing potentiometer as ior measuring the electromotive iorce developed in a thermoelectric system, in which the control oi a'condition is eiiected by the joint operation oi the said electrical circuit and potentiometer.

Reierring to'the drawing, l0 and II designate the "hot" and cold junctions, respectively, oi a thermoelectric couple whose total thermoelectromotive iorce it-is desired to measure by means 7 oi the position oi a sliding contact i2 engaging 'a slide-wire element II, constituting a potentialv divider and carrying a current of constant value derived from a battery N. The condition oi balance ls detected in the usual manner by a galvanometer. I! connected in series with the thermoelectric circuit andthe sliding contact l2; and the position oi the contact may be indicated by a pointer ll traversing a graduated scale II.

A reversible motor l8, having mechanically operative connection to the contact I2 ior traversing the same over the slide-wire element It, is provided with two windings t0 and 20, adapted to exert rotative influences in opposite senses.

For example, energization oi the winding i9 tends to operate the motor to move the contactor in a sense to increase the proportion oi the slide-wire potential included in thecircuit in opposition to the thermbelectric'iorce to be measured and developed by the thermocouple l0; and the winding tends to operate the motor in a sense to decrease the amount of said voltage. The motor I! is also, preferably, oi the design in which simultaneous energization oi both windings to an equal extent will not only produce a zero rotative influence, but will provide definite dynamic braking, thus tending to bring the motor quickly to rest aiter operation in either direction.

v It will be understood that a condition oi unbalance in the potentiometer circuit, resultingirom changes in the condition under control, causes a flow oi current in the galvanometer coil and a corresponding deflection, the direction oi the deflection depending on the direction of the unbalance. The means for obtaining primary response to deflections oi the galvanom'eter ll comprises a pair oi identical electrical oscillatory circuit systems 2| and 22, arranged as follows: Oscillating circuit system 2i includes a threeelement thermionic tube 23 which has connectedbetween its grid and its cathode a coil 24-, mechanically juxtaposed to, and having a degree of magnetic linkage with, a coil 25 connected in the output circuit oi the'same thermionic tube.

Carried by the galvanometer i5, and moving when the same is deflected, is a vane 26 oi lightweight, electrically-conducting material, the same being adapted to intersect the common magnetic circuit oi the coils 24 and 25; and, by virtue oi eddy currents induced in the vane when currents oi oscillatory frequency are 'flowing in either of said coils, said vane is designed to aiiect the magnetic linkage or mutual inductance between the same, thereby producing a shielding eiiect. .According to principles well known to the art, the magnitude of the oscillations in the.circuit containing the coils 24 and 25 will be aiiected by the extent to which the vane 28 shields oneirom the other; and the nature oi change oi this effect in response to a given movement oi the vane will also depend upon the relative tions of oscillation there will be developed between the grid and the cathode of tube 23 an alternating potential of the oscillation frequency; and, because of the fact that current will pass in one direction from cathode to grid when the grid is at a positive potential with respect to the oathode, a unidirectional pulsating current will flow in the grid circuit. A grid-leak resistance 28, connected in series with the grid of tube 23, will have developed across its terminals a unidirectional potential, whose average value will bias the tube negatively, causing a decrease in the value of plate current from the value which otherwise might flow. A suitable condenser 29, in parallel with the resistance 28, will act to bypass the pulsating component of the current in the grid circuit and retain its charge over the half cycle where no grid current flows; and a suitable condenser connected between the cathode of the tube and the free end of the coil 25 will provide a path for the oscillatory component of current in the plate circuit of the tube and in the coil 25.

The other oscillating circuit system 22 is in all respects identical with the circuit system 2i, and includes a three-element thermionic tube 3|, having in its grid and plate circuits respectively coils 32 and 33 with a degree of magnetic interlinkage, and in whose common field may pass the shielding vane 26 when deflected away from its association with the coils 24 and 25. A tuning condenser 34 is connected in parallel with the coil 32 for establishing the period of oscillation; and there is provided a grid-leak resistance 35 having a parallel condenser 36, together with a bypassing condenser 31 between the plate circuit and the cathode of the tube 3|.

Power for the operation of the tubes and associated oscillating circuits is derived from a transformer 38 having a single primary winding 39 connected to a suitable source of alternating current supply. The secondary provides a filament current winding 40 and a plate voltage winding 4|, operatively connected, as shown, to the filament and plate systems of the oscillating circuit systems 2| and 22, together with an additional filament current winding 42 and a plate voltage winding 43, the purpose of. which will hereinafter be set forth. The filaments of the tubes 23 and 3i are connected in parallel to the winding 40 of the transformer 38.

A transformer 44 is provided with a primary winding 45 and with a secondary winding 48, each winding being tapped at its mid-point. The mid-point of the primary winding 45 is connected to one terminal of the plate voltage winding 4| of the transformer 38, the other terminal of said transformer winding being connected to the circuit supplying the filaments 23 and 31;

Operation of the balancing motor I8 is effected through the medium of a relaycircuit system 41, including a'pair of similar grid-controlled rectiflers (thyratrons) 48 and 49, having their cathode heating circuits supplied with current from the secondary winding 42 of the transformer 38, and their grids connected through suitable grid-leak resistors 58 and 5i to the leftand right-hand ends, respectively, as seen in the drawing, of the secondary winding 46 of. the transformer 44, thus constituting a network adapted to utilize the well-known threshold effect characterizing grid-controlled rectifiers of this class, and providing trigger circuits for the same. One terminal of the secondary winding 43 is connected to the mid-point of the secondary winding 46, and, through a biasing resistor 52, to the common cathode circuit of the thyratron tubes. It will be obvious to those versed in the art that while the grid-controlled rectifiers provide a preferred construction, a similar performance may be obtained by the substitution of other types of thermionic trlodes for the same, whereupon the trigger or threshold effect may be replaced by an amplifying effect which may be utilized in a like manner, as here inafter set forth.

The plates of the tubes 48 and 49 are connected to the opposed windings I9 and 2!) respectively of the motor I8, and through said motor to the terminal of transformer Winding 43 -to afford respective control circuits for the said motor. The value of the resistor 52 may be either such that the tubes 48 and 49 will be biased to a point just above cut-off, when, under balanced conditions, no current will flow in either winding of the motor l8; or it may be such that the rectifying tubes are biased to a point just below cut-off, when, under balanced conditions, the respective motor windings will be carrying currents of equal and opposed influence, so that the motor will remain at rest and will have dynamic braking. In general, operation of the motor l8 will be determined by the predominance of one over the other of currents flowing in its respective windings.

The performance of the control-system under operating conditions may be described as follows: Assuming first a condition of balance to exist in the thermocouple circuit, the sliding contact l2 being in such a position on the slidewire 13 that the thermoelectromotive force in the thermocouple circuit is exactly opposed by the selected portion of the battery l4, no current will be flowing in the galvanometer l5, and the shielding vane 26 will rest in a neutral zone just outside the fields of the two feedback coil units of the two oscillating circuit systems. Under this condition both the circuit systems 2| and .22 will oscillate when their respective coils are properly polarized and, when properly adjusted, will have equal currents flowing in their plate circuits, which currents, passing through the two opposed sections of the primary winding 45 of the transformer 48, will neutralize their magnetic effects, with a resulting zero voltage in the secondary winding. Thus, the grid-controlled tubes 48 and 49, being normally biased alike, as hereinabove set forth, will produce no unbalance current in the windings of the motor l8, which will consequently remain at rest.

Assume now an increase in the total electromotive force in the thermocouple circuit, due to a rise in the temperature to which the junction I0 is exposed. This will produce a condition of electrical unbalance in the circuit, which will cause a current to flow in the galvanometer l5, whose polarity has been selected such that the vane 26 will be deflected toward the left as seen in the diagram, shielding the coil 24 from the coil 25. With these coils of polarity to establish the required phase relations, the result will be to reduce or check the oscillations of circuit systern 2i with a corresponding reduction in the biasing potential between the grid and cathode of tube 23, and a consequent increase in plate current fed from the transformer winding 4|. The half-wave plate current in tube 23 will flow in the left-hand half of the winding 45 of transformer 44, upsetting thereby the normally balanced condition, and inducing an alternating voltage in its secondary winding 46. The voltage across the left-hand hall of the winding 48 will 23 and ll.

be 180 out 01' phase with the voltage across the right-hand-hali of winding 46, and one of these voltages will be in phase with the plate voltage of the thyratron tubes 48 and 4!, since they are fed from the same transformer as the tubes In'one oi the tubes 48' and 49, therefore, the grid will be negative with respect to the cathode during the time that the plate is positive with respect to the cathode, thus producing a negative bias and preventing plate current flow-it being understood thatplate current can flow only when the anode is positive with respect to the cathode. thyratron", being at a, potential 180 out 0! phase with the aforementioned grid, will be negative with respect to the cathode when the plate is negative with respect to the cathode and positive when the plate ispositive, so that plate current will flowl The winding 19 of the motor II will thus be energized, causing the motor to operate and to move the sliding contact I! along the slide-wire ll in a sense to increase the potential opposed to that of the thermocouple. system, and to-restore a condition of balanced potential in the thermoelectric circuit, reducing to zero the current in galvanometer I5, allowing the vane 28 to return to its neutral position, andnormal oscillatory conditions to be restored in the circuit.

In a similar manner, should there take place a f lowering of the temperature to which the thermocouple III is exposed, causing in the thermoelectric circuit a change of potentialtending to'deflect the galvanometeitoward the right, the vane 26 will tend to reduce oscillations in the circuit system 22, allowing current to flow in the right hand halfoi the winding 45 of transformer 44,

to which the "null" method of measurement may be adapted.

and eilecting the bias tube 49 to cause the motor.

II to act inasense to decrease the potential opposed to the thermoelectromotive iorce, again tending to restore balanced electrical conditions throughout the potentiometer system.

Thus, 'it will be apparent that the position of the sliding contact I! along the slide-wire I3, and therefore oithe pointer II on the graduated scale l1, becomes a measure oi the total thermoelectromotive iorce in the thermoelectric circuit,

and thus of the temperature to which the hot junction ill is exposed. It is obvious that the pointer l8 traversing the scale I! may be replaced by any of the well-known toms of recording device, embodying a pen ;or stylus traversing a ,travelling'paper chart (not shown), thus providing a continuously recording potentiometer.

It will further be obvious that the application oi the balancing combination need not berestricted to its use with a potentiometric type of measuring circuit, but may with equal facility be associated with any type 0! electrical bridge circuit, or with any tom 01' mechanical instrument The grid of the other position, the said circuit systems and the vane 20 may be so modified, asby reversing one o! the coils and by proper adjustment of the circuit con.

'stants, that the circuit systems 2i and 22 will be normally non-oscillating, either one or the other being thrown into oscillation upon deflection oi Moreover, the invention is not restricted to the particular type of the oscillation systems shown; nor is it restricted to the thyratrcn relay system, as standard electromagnetic relays may be utilized in the usual manner. Also, the motor V utilized for restoring the balance in the potentiometer circuit may be either oi the induction type or of the commutating type.

I claim: 1 1. In a system of control and/or measurement and including a null method measuring system subject to unbalance, with electric motor means for reestablishing a balance in said system, and a sensitive member responsive to conditions of un-' balanceyelectronic means ailordingan oscillatory circuit; relays means controlling operation of the said motor. means, together with electrical connections between-said relay means and said circuit including means for rendering said relay means responsive to variations in"th'e oscillatory condition of said circuit; andmeans to change the intensity of the ieed-back'in said oscillatory circuit and actuated by the said sensitive mem-- electric motor for reestabjlishing a balance in said system: electronic means affording an oscillatory circuit; relay means controlling operation of the said motor means, together with electrical connectioris between said relay means and said circult including means for rendering said relay means responsive to variations in the oscillatory condition of said circuit; and means controlled 'by the said galvanometer to change the intensity of the feed-back in said oscillatory circuit to efiect operation of said relay means.

3. The combination with a null method measuring system subject to unbalance and including electric motor means for establishing a balance in said system, and a sensitive member respon'sive to conditions of unbalance: of a pair of electiicai'networks each including thermionic means provided with input and output circuits, means for providing inductive interlinkages between the input and output circuits in each of said networks, whereby oscillatory conditions may be maintained independently in each network, and a relay means including a pair of thermionic devices, each provided with a trlggericircuit and a control circuit, the latter being adapted to actuate said motor means in respectively oppositc senses, and said trigger circuits being rapectively responsive to changes in the non-pulsating component of plate current, which varies according to the oscillatory conditions of said pair of networks, together with means actuated by said sensitive member to change the intensity of the feed-back in each 0! said networks.

4. The combination with a null method measuring system subject to unbalance and includinz lil electric motor means for establishing a balance in said system, and a sensitive member responsive to conditions of unbalance: of a pair of electrical networks each including thermionic means provided with input and output circuits, means for providing inductive interlinkages between the input and output circuits in each of said networks, whereby oscillatory conditions may be maintained independently in each network, and a relay means including a pair of thermionic devices, each provided with a trigger circuit and a control circuit, the latter being adapted to actuate said motor means in respectively opposite senses, and said trigger circuits being respectively responsive to changes in the non-pulsating component of plate current due to variation in the oscillatory conditions of said pair of networks, together with means actuated by said sensitive member to change simultaneously the intensity of the feedback in each of said networks.

5. The combination with a null method measuring system subject to unbalance and including electric motor means for establishing a balance in said system and a sensitive member responsive to conditions of unbalance: of a pair of oscillatory circuits including amplification means, electromagnetic shielding means whereby said circuits are rendered oppositely responsive to deflections of said sensitive member to change the intensity of ieed-back in said circuits, and a relay system including thermionic devices responsive to the oscillatory conditions of said oscillatory circuits and adapted to command response of said motor means.

6. The combination with a null method measuring system subject to unbalance and including electric motor means for establishing a balance in said system and galvanometer means responsive to conditions of unbalance: of a pair of oscillatory circuits including amplification means, electromagnetic shielding means whereby said circuits are rendered oppositely responsive to deflections of said galvanometer means to change the intensity of feed-back in said circuits, and a relay system including thermionic devices responsive to the oscillatory conditions of said oscillatory circuits and adapted to command response of said motor means.

7. The combination with a null method measuring system subject to' unbalance and including electric motor means for establishing a balance in said system and a sensitive member responsive to conditions of unbalance: of a pair of thermionic tubes affording oscillatory circuits and output circuits, electromagnetic shielding means whereby said oscillatory circuits are rendered oppositely responsive to deflections of said sensitive member to change the intensity of feedback in said circuits, and a relay system including thermionic devices associated with the respective output circuits and responsive to the oscillatory conditions of said oscillatory circuits and adapted to command response of said motor means.

8. The combination with a null method measuring system subject to unbalance and including electric motor means for establishing a balance in said system and a sensitive member responsive to conditions of, unbalance: of a pair of electrical oscillatory circuits including thermionic devices, electromagnetic shielding means actuated by said sensitive member to change the intensity of feed-back in said circuits, and amplification means, subject to variations in the output current of said thermionic devices, to command response of said motor means.

FRED B. MACLAREN, JR. 

